Strictly speaking, the Umm al-Qura calendar is intended for civil purposes only. Their makers are well aware of the fact that the first
visual sighting of the lunar crescent (hilāl) can occur up to two days after the date predicted by the Umm al-Qura calendar.

Since 1419 AH (1998/99 CE) several official hilāl sighting committees have been set up by the government of Saudi
Arabia to determine the first visual sighting of the lunar crescent at the begin of each lunar month. However, the religious authorities of
Saudi Arabia also allow the testimony of less experienced observers and thus sometimes announce the sighting of the lunar crescent on an
evening when none of the official committees could observe the lunar crescent or even on an evening when the lunar crescent actually set
before sunset.

This often leads to confusion when the dates of important religious events in Muḥarram, the month of fasting (Ramaḍān) or the
month of pilgrimage (Dhu ʾl-Ḥijja) are changed. In nearly all of these cases, a retrospective analysis indicates that these
extremely early reports of the lunar crescent are impossible and were based on false sightings. Most of these false sightings were probably
caused by a bright star or planet (such as Venus) or an airplane contrail viewed near to the western horizon.

A study of 42 reports of sightings of the Ramaḍān new moon, as announced by the High Judiciary Council of Saudi Arabia
(Majlis al-Qadāʾ al-Aʿlā) between 1962 to 2001 (1381 AH to 1422 AH), confirms that more than half of these
were too early and based on false sightings of the lunar crescent (Kordi, 2003). A similar result was reached by a later study covering
the years 1380 AH to 1425 AH (Gadi, 2007).

The following table compares the Umm al-Qura dates for the begin of the months Muḥarram, Ramaḍān, Shawwāl and
Dhu ʾl-Ḥijja with the dates announced by the High Judiciary Council of Saudi Arabia (source:
Fatwa-Online):

In some cases the advancement of the month can result in a month length of 31 days which is awkward as Islamic tradition only allows
for month lengths of 29 or 30 days. In such cases one of the days in the month is reckoned twice. For instance, both Friday 28 December
and Saturday 29 December 2007 were reckoned as 19 Dhu ʾl-Ḥijja 1428 AH.